1. Field of the Invention
The present invention relates to a process for upgrading lubricating oil stocks. More specifically, the process relates to solvent treating a lubricating oil stock with a solvent comprising a mixture of n-methyl-2-pyrrolidone (NMP) and phenol, which may also contain a minor amount of water, thereby removing at least a part of the aromatic, sulfur and nitrogen-containing constituents contained in the lubricating oil stock. Still more specifically, the process relates to improvements in the recovery of solvent from raffinate and extract phases formed during the extraction operation.
2. Description of the Prior Art
In prior art extraction processes utilizing phenol and/or phenol-water solvent systems, several process difficulties have been encountered. Typical of such prior art extraction processes is that disclosed in U.S. Pat. No. 3,329,606, which discloses a process comprising introducing a feedstock into an extraction column and contacting the feed therein with phenol. The phenol is introduced into the top of the tower and contacted therein countercurrently with the upward rising feedstock. Simultaneously, phenolic water is introduced near the bottom of the column. The overall object of the process is to improve the efficiency of the phenol/water solvent extraction system. This is accomplished by providing internal reflux near the bottom of the column by increasing the selectivity of the solvent at that point.
Another process which utilizes phenol or phenolic water is that described in U.S. Pat. No. 2,923,680. Specifically, the process described therein is directed to improvements in the recovery of phenol from raffinate and extract products obtained when subjecting a lubricating oil stock to a phenol extraction treatment. The process comprises heating the raffinate and extract phases in separate heating zones to temperatures ranging between about 450.degree. and 750.degree.F., thereby vaporizing a major portion of the phenol present in the respective phases. Residual quantities of the solvent are thereafter removed by contacting the raffinate and extract phases with a non-aqueous stripping gas. This separation process represents an improvement over those described in the prior art which generally utilizes steam as the stripping agent. Steam stripping has the disadvantage of getting water into the extraction unit, which water must then be removed to keep the proper solvent composition. The difficulty apparently involved in removing the residual water results in an unattractive economic configuration.
As indicated supra, the phenol/water solvent systems have led to difficulties in solvent recovery and solvent aromatic constituent selectivity. Examples of such difficulties include the relatively slow settling rate of the raffinate and extract phases in a multi-tray extraction zone when the extraction solvent is aqueous phenol due to the low interfacial surface tension of the two phases, and the relatively high viscosities at the operating temperature of the extraction zone. This often leads to inter-tray entrainment, and also to entrainment of oil feed in the extract phase leaving the tower bottom. In this manner, raffinate yield is lowered. Additionally, post-extraction hydrofining operations, necessary to improve raffinate product color, must be run at relatively high severity in order to achieve color specifications. Furthermore, relatively large amounts of water are normally required to be added to the phenol in order to obtain the desired solvent selectivity (typically 5-10 percent). The high water content is detrimental in view of the large heat input needed for solvent recovery (water has about 5 times the latent heat of vaporization per pound of phenol). Additionally, the separated phenolic water solvent system condenses as a two liquid phase system, i.e., wet phenol and phenolic water. This makes it more difficult to control the critical water content in the extraction solvent, since the two phases do not readily separate and are usually pumped away as an emulsion of variable water content.
Other solvents have been used in extraction processes in an attempt to avoid the above-mentioned problems. Typical of such prior art processes include that described in U.S. Pat. No. 3,476,681 wherein the solvent comprising n-methyl-2-pyrrolidone and water is used to extract aromatic constituents from a lubricating oil stock. The process utilizes a rather complex separation configuration to remove solvent from extract and raffinate phases and it is therefore economically unattractive. Furthermore, NMP is not as efficient as phenol in removing undesirable organic nitrogen compounds from the raffinate.
Other prior art processes, such as that described in U.S. Pat. No. 3,496,069 have utilized solvent mixtures such as n-methyl-2-pyrrolidone, water and phenol in extraction processes. The 3,496,069 process, however, is directed to the preparation of high purity alkadienes of 4 to 5 carbon atoms containing low quantities of alkanes and is not concerned with lube oil stock preparation. In addition, U.S. Pat. No. 3,556,987 and U.S. Pat. No. 3,617,495 disclose the use of n-methyl-2-pyrrolidone/phenol solvents for use in naphtha (b.p. 100.degree.-400.degree.F) extraction, again bearing no relationship to lube oil stock preparation
In view of the above-mentioned problems in the preparation of high quality lubricating oil stocks, it would be desirable to develop an extraction process for the preparation of high V. I. lubricating oils of good color, utilizing a solvent system that avoids the abovementioned difficulties existing in present prior art processes, and further, permits rapid settling of extract and raffinate phases in the extraction tower, eliminates or substantially reduces the severity of the post-extraction hydrofining operation, and the amount of water required in the solvent mixture.